1. Field of the Invention
The present invention relates to improvements in cutting tools using a sintered body comprising mainly cubic boron nitride (hereinafter CBN sintered body) as the substrate. A CBN sintered body means a sintered body of polycrystalline CBN and is hereinafter referred to as PCBN. In particular the invention relates to a coated PCBN cutting tool improved in wear-resistance and high accuracy machining.
2. Description of the Related Arts
Methods to coat a surface of the PCBN cutting tool with a variety of wear-resistance layers such as TiN so as to improve the wear-resistance of a CBN sintered body have been proposed (i.e. JP-A-1-96083 and JP-A-1-96084). It has also been proposed to roughen the surface of the substrate (CBN sintered body) considerably by ion etching and then to coat the roughened surface, in order to improve the adherence between the substrate and the coated layers, and the durability of the coated layer (i.e. JP-A-7-18415).
However when using a roughened surface of the substrate to improve adherence, the coated surface is also roughened and liable to peel due to increased cutting resistance.
Surface roughness of the work or workpiece depends on the shape of the end cutting edge boundary, because the shape of end cutting edge boundary is reproduced on the work. FIG. 3 is an enlarged ground plan seen from the direction of the rake face 31 when work 30 is cut by an insert.
The arrow shows the feeding direction of the insert at cutting in FIG. 3. Herein the end cutting edge boundary 33 is a portion of the cutting edge which forms the finished surface of the work. The broken line in FIG. 3 shows wear of the insert. Number 34 is the side cutting edge boundary.
When a roughly coated insert is used, the surface roughness of the work becomes rough from the early cutting stage, since the surface of the insert is reproduced on the work. Particularly when the end cutting edge boundary is unevenly worn and notch wear develops, the finished surface becomes rough within a short cutting time.
Consequently, the main object of the present invention is to provide a coated CBN sintered body for an insert which can produce high precision and high quality machining of work for a remarkably long tool life compared with the conventional insert.
The present invention is based on the knowledge that the smooth wear at the end cutting edge boundary controls increasing surface roughness of the work, enabling long tool life and high precision machining. The object of the present invention is accomplished by a coated PCBN cutting tool which has a prescribed coated material, thickness and surface roughness.
The present invention relates to a coated PCBN cutting tool and a CBN sintered body comprising 35% to 85% by volume of CBN. The hard coated layer consists of at least one element selected from a group consisting of the group IVa, Va, VIa elements of the periodic table and Al, and of at least one element selected from a group consisting of C, N and O. The thickness of the layer is preferably 0.3 xcexcm to 10 xcexcm. The surface roughness of the coated surface is not more than 0.2 xcexcm Ra, which is the center-line mean roughness.
Since large particles called droplets may be contained in the hard coated layer, Ra is preferable to estimate the surface roughness of such hard coated layer. The inventors have found that the end cutting edge boundary is worn smoothly when the hard coated layer is smooth. The preceding term xe2x80x9csmoothly wornxe2x80x9d means controlling the flank wear and notch wear at the end cutting edge boundary.